Noninfectious virions generated by superinfection of murine S plus L minus cells and by clones of cells infected by restricted leukemia virus will be further analyzed biochemically in an effort to correlate the biological and biochemical defects in these particles. N- and B-tropic MSV infection of cells will be studied biochemically as well as biologically to define with greater precision where, in the life cycle of the RNA tumor virus, Fv-1 restriction acts.